Garden Calendar: Pervious building materials

The patio on the west side of the Douglas County Bank building located at 1444 Kasold Drive was built with a permeable paver system, which is beneficial in reducing stormwater runoff and in filtering the water as it soaks through the ground.

The patio on the west side of the Douglas County Bank building located at 1444 Kasold Drive was built with a permeable paver system, which is beneficial in reducing stormwater runoff and in filtering the water as it soaks through the ground.

Newer permeable pavestones have a lug on the side that keeps installers from spacing them too close to other stones — similar to the way tiles are spaced. Stones are still close together though, typically with a 1/8-inch-wide void. Gaps are then filled with small angular gravel: 1/4 to 3/16 of an inch in diameter washed gravel, to be exact.

Almost every time a new building goes up or a new parking lot is built, the earth loses a little space where rainfall could be soaking back into the soil. With nowhere else to go, water runs off the impervious surfaces and travels through storm sewers and ditches on its journey to the river. Also, as the water flows, it picks up trash and pollutants and carries them into rivers and streams.

For many years, the standard for stormwater has been to get it off the streets as quickly as possible to prevent flooding. I still remember the first time I saw cars floating down 23rd Street after a quick, heavy rain. Sometimes storm sewers and ditches simply cannot handle the amount of water that needs to be moved.

With more buildings and paved surfaces than ever before, engineers and environmentalists are exploring other options. Green roofs, rain gardens, bioswales and retention ponds are a few of the ways water can be dealt with onsite instead of letting it become someone else’s problem.

Sometimes a flat, smooth surface is needed, though, so a bioswale is not exactly appropriate. That’s why researchers have also been working to develop pervious concrete, asphalt and paver systems. Some of the early systems worked OK for infiltration but clogged over time or were less aesthetically pleasing than most people preferred.

The good news is that the systems are getting better and looking better. I hope they will grow in popularity. That’s why I was happy to see that Douglas County Bank chose a permeable paver system over an impervious surface for their new patio at the Bob Billings Parkway and Kasold Drive location.

The permeable system came highly recommended by Clay Phillips, the architect from BG Consultants who was working on the project.

“I think they’re a good product,” Phillips says. “The water has a chance to go back into the groundwater system.”

Phillips, who has used permeable paver systems in previous projects, notes that permeable systems have come a long way since their introduction.

“Some of the systems are not as pervious as these. If you’ve seen the ones with bigger gaps, with grass growing in them — they don’t work as well as these.”

Pat Slabaugh, executive vice president at Douglas County Bank, is enthusiastic about having something that doesn’t hold water but also does not add to the amount of impervious surface.

“I really think it was the right thing to do, and I like the look of them,” Slabaugh says.

The main difference between the newer permeable paver systems and the older systems is the look. As Phillips mentioned, many had large gaps that were meant to be filled with grass or other plants. It was a poor environment for plants to grow and the systems generally did not work the way they were intended.

Brad Minnick, sales representative with Capitol Concrete who has worked with the paver systems, also says the system is much improved. He also tells me there is a bit of a misconception about the permeability of regular pavers and flagstone.

“With normal pavers, the gaps are narrower and filled with a polymer sand that sets up almost like concrete,” Minnick says. “Even with the porous concrete and pervious asphalt, the systems become plugged over time, and there’s no mediation system.”

Newer permeable pavestones have a lug on the side that keeps installers from spacing them too close to other stones — similar to the way tiles are spaced. Stones are still close together though, typically with a 1/8-inch-wide void.

Gaps are then filled with small angular gravel: 1/4 to 3/16 of an inch in diameter washed gravel, to be exact.

Permeable paver systems also filter and clean storm water, Minnick says. “Colorado State has done studies that show the heavy metals that leach off parking lots — and how much the permeable systems filter the water.”

Some cities, including Portland, Ore., have gone as far as replacing streets with permeable paver systems.

If you are thinking about a new patio or driveway, Minnick says the system is easy enough for the average do-it-yourselfer to install. And, you can feel good about the environment while doing it!